FIG. 1 is a simplified diagram of a microwave electron tube comprising essentially three main subassemblies, namely an electron gun 12, a microwave structure 14 and a collector 16.
The electron gun 12 comprises a cathode 18 that generates an electron beam 20 in the microwave structure 14, where the electron beam 20 interacts with an electromagnetic wave created in the microwave structure. More precisely, the electron beam gives up some of its energy to the electromagnetic wave.
The collector 16 thermally dissipates the kinetic energy of the electrons of the beam 20 that remain after interaction with the electromagnetic wave.
The electrons emitted by the cathode are accelerated by a voltage V0 applied between the cathode and the anode of the tube and are characterized in a current I0.
The microwave structure is composed of resonant cavities and drift tubes in the case of klystrons, and of a helix or coupled cavities in the case of a TWT.
The microwave structure of the TWT includes an input window 22 on the side facing the gun of the tube, in order to inject the power Pe to be amplified in the structure, and an output window 24, on the side facing the collector, for extracting the amplified output power Ps.
The gain G=10log10 (Ps/Pe) is around 40 to 50 dB and the interaction efficiency ηi=Ps/V0I0, which is generally between 30 and 60%. These input and output windows are dielectric members, usually made of alumina, which transmit, almost without loss, in the operating frequency band of the tube, the input microwave power Pe, into the structure, and the output power Ps, to the outside of the structure, depending on the case, while isolating the inside of the tube, which is under vacuum (residual pressure ≦10−7 torr), from the external atmosphere.
Another likewise essential subassembly of the tube is a magnetic circuit 40 (see FIG. 1) that surrounds the microwave structure 14, comprising an electromagnet or permanent magnets associated with pole pieces for conducting the magnetic flux into the electron beam 20 which is thus focused, that is to say maintained at a small and approximately constant diameter. This magnetic circuit is external to the vacuum chamber of the tube, except sometimes for certain pole pieces.
An ion pump 42, indicated in FIG. 1, is used to maintain the vacuum inside the tube-this pump is not always necessary.
The collector 16 is a hollow cylinder, as indicated in FIG. 1. The electrons from the beam bombard the internal walls 44 of the collector 16, which heat up. The heat is then extracted via the outer walls of the collector, which are cooled, depending on the power densities in question, by forced air, by water circulation or by radiation.
The collector is at the potential of the body of the structure 14 of the tube, that is to say at ground potential, the cathode being at potential −V0.
The collector 16 may be directly attached to the body 14, as indicated in FIG. 1. The collector may also be electrically isolated from the body, but connected to the latter via an external electrical connection.
FIG. 2 shows a partial view of a TWT comprising a microwave structure 50 having coupled cavities 52 and a collector 58 attached to the microwave structure 50 and electrically isolated from the body of the tube, and especially from an upper pole piece 60, via an annular insulator 62. The electron beam 20 output by the microwave structure penetrates the collector 58 via an aperture 64. Electrons following various paths 66 are collected by the internal walls 68 of the collector.
It is often necessary to separately measure the current Ib of the electrons that are intercepted by the microwave structure and the current Icoll of the electrons that reach the collector. These two currents have very different amplitudes, often with an Ib/Icoll ratio of a few %, or even 1% or less.
To do this, the collector is isolated from the body by the insulator 62, for example made of a ceramic, often alumina (see FIG. 2). FIGS. 3a and 3b show schematically the electrical connections of the various elements of the tube of FIG. 1 to the power supply AL 70. It is the body of the tube which in general is connected directly to ground G, for practical reasons, as it is of course connected to the external installation via the input and output waveguides, often via the armature of the electromagnet, and sometimes via the systems for tuning the cavities, thermal probes. The hydraulic connections for the collector, when they exist, must therefore be sufficiently insulated to force the current Icoll not to follow them as return path, via ground, back to the + pole of the power supply.
The collector is isolated from the body by an annular ceramic piece 62 (FIG. 2), or in general by any other insulator, which fulfils several important roles:                electrical isolation between body (or pole piece) 60 and collector 58;        sealing and maintaining the vacuum inside the tube;        mechanical strength, in order to keep the collector firmly in place on the body, despite certain vibration that occasionally arises from the cooling system and despite the knocks that it may receive when being transported and installed.        
However, this body 60/collector 58 isolation appears, from the microwave viewpoint, as a true radial line, itself composed of several lines of different impedances Z1, Z2, . . . Zi in series.
FIG. 4 shows a detailed view of the space Wg for coupling between a body 80 and the collector 82 of a microwave tube. This space is shown as a series of lines of impedances Z1, Z2, Z3 in series between the inside and the outside of the tube. The value of these impedances is related to the geometrical characteristics (h, d, etc.) of the lines and to the presence or absence of a ceramic insulator (∈0, σ). The reader may refer to the work “Fields and waves in communication electronics” by Ramo, Whinnery et al. (published by John Wiley & Sons).
It follows that if electromagnetic energy is present at the input Ecoll of the collector, it may be coupled to this radial waveguide and can radiate (Pr) to the outside.
The presence of electromagnetic energy at the input of the collector may be due to leaks from the output cavity (or from the helix), or else the drift tube connecting it to the collector, i.e. to the cutoff at the operating frequency F and generally at 2F. However, this tube is often too short, therefore allowing evanescent mode transmission.
This electromagnetic energy may also arise from one of the many resonances of the collector that are excited to F, 2F, etc. by the electron beam, again slightly modulated.
In other words, the radial waveguide can present to the electron beam an impedance Zed sufficient for the beam, again slightly modulated, to give up thereto microwave energy at a low but not insignificant level, which is then radiated to the outside via the radial waveguide between body and collector.
Now, the specifications often impose a very low level of microwave loss, for example Pr<0.1 mW/cm2 at 10 cm over the entire external surface of the tube.